Will wearing dreads in a pony tail prevent locking?
For the most part it's fine to tie your dreads back even when they are new, but there are a couple things to look out for:
1) if the band you're using to hold them back is very tight and always in the same place, pressing on the same dreads, it will prevent knots from locking in that area of the dread. Using a wider hair elastic to pull them back or tying them with a bandana can spread out the pressure and prevent any problems.
2) Some slack in the dread is necessary for the dreads to shrink up and lock while they are new and for new growth to lock as the hair grows - if a pony tail is tight and worn all the time it's keeping everything stretched out and it could make it very difficult for the new growth to have the slack it needs in order to dread. You can slide the dreads that feel tight out of the band, toward the root a little to relieve the tension. Ultimately it's clockwise rubbing that will help the new growth lock.
Also if you experience itching under the pony tail it may be because there is too little air circulation. This usually isn't a problem unless you leave them tied back tightly for a long time. If it feels like the scalp is moist and or itchy under the pony tail you should wash your scalp well and dry it with a hair dryer.